1. Field of the Invention
This invention relates to a volatile gas detector and particularly to a portable photo-ionization detector (PID).
2. Description of Related Art
Photo-ionization detectors (PIDS) can detect volatile gases. FIG. 1 shows a conventional portable PID 10 that includes an ultraviolet (UV) lamp 12 and an ionization chamber 14. UV lamp 12 produces UV light including UV photons having energy up to 8.4 electron volts (eV) or more. The UV photons pass through an optical window 16 into ionization chamber 14. In ionization chamber 14, the UV photons collide with and ionize volatile gas molecules having ionization potentials below the energy of the photons, creating ions and electrons.
PID 10 further includes an ion detector 18 having a pair of electrodes 20 and 22, which are typically made of a metal. Ion detector 18 has a high voltage (150 V or more) applied across electrodes 20 and 22 to generate an electrical field. In particular, electrode 22 is biased to a high voltage to attract negatively charged particles (electrons) and repel positively charged particles (ions), and electrode 20 is grounded to collect the positively charged particles (ions). The movement of the ions to electrode 22 produces a current, from which the concentration of the volatile gas can be determined. More specifically, the magnitude of the current depends on the number of ions produced from the collisions between volatile gas molecules and UV photons. Accordingly, the magnitude of the current depends on the concentration of ionizable volatile gas molecules and the intensity of UV light in ionization chamber 14. If the UV light intensity is constant, a measurement of the current directly related to the concentration of ionizable gases.
During use of PID 10, a gas sample in ionization chamber 14 can contain air mixed with one or more volatile gases that have ionization potentials lower than the maximum energy of the UV photons from UV lamp 12. PID 10, which has a single ion detector 18, measures ion current and the total concentration for the ionizable gases of all types in the sample. PID 10 cannot determine the concentrations of individual gases in the gas sample.
U.S. Pat. No. 5,393,979, which is herein incorporated by reference in its entirety, discloses a PID that includes multiple single channel PIDs that measure the concentrations of different types of gases in a gas sample. For instance, the PID may include three UV lamps having different maximum photon energies of 9.8, 10.2, and 11.7 eV and an ionization chamber including three ion detectors respectively in front of respective UV lamps. When a gas sample flows through the ionization chamber, each of the UV lamps, which are arranged in tandem, ionizes only the gases having ionization potentials below the maximum photon energy of the lamp, and the associated ion detector measures a current that the lamp generates from the gas sample. The three measured currents from the ion detectors differ from one another if the gas sample contains gases that can only be ionized by UV light from some of the lamps. The concentrations of gases having ionization potentials below each photon energy can be determined from the respective measured currents.